Graft polymers, formed between epoxy resins and addition polymers, generally formed from monomer including an acrylic acid, are suggested in these earlier applications and in certain prior art for use in coating compositions.
In the art prior to these applications, it appears that the so-called graft polymer has been an adduct formed by an esterification reaction between an acidic addition polymer and an epoxy resin. The techniques for making epoxy-based resins water-dispersible, through inversion with amines, and for cross-linking them with added aminoplast, are well known. None of the prior art adduct products has been fully satisfactory. The adducts and coating compositions formed from them do not have good resistance to water, because the adducts are susceptible to hydrolysis.
In application Ser. No. 685,246, there are disclosed processes for making curable resinous compositions and coatings that are unique. The coating compositions are particularly valuable when formulated for use as sanitary coatings, that is, as coatings for cans for edible materials. Such can coating compositions are easy to apply in existing equipment, have excellent shelf stability, and after application and curing have excellent functional properties as coatings and are bland and so do not impart any flavor to the food or beverage in the can.
In one embodiment, that application discloses a process for preparing a curable resinous composition having an Acid Number of at least 30, by reacting together at 90.degree. C. to 130.degree. C. an aromatic diepoxide resin having a molecular weight above 1,000 and addition polymerizable monomer of which from 10% to 80% by weight is an acrylic acid, the diepoxide resin being present in sufficient quantity to provide from 30% to 90% by weight of the initial reaction mixture, in the presence of a free radical initiator of the benzoyl peroxide type. During the reaction there is simultaneous addition polymerization of the monomer through its ethylenic unsaturation and grafting of addition polymer to the diepoxide resin. The graft polymr is characterized by the substantial absence of hydrolyzable functional groups. The acid functionality of the reaction mixture is sufficiently high to effect stable dispersion of the product in a basic aqueous medium.
In a preferred embodiment, an aromatic diepoxide, a polyglycidyl ether of bisphenol A, is reacted with a mixture of additional polymerizable monomers containing a major amount of methacrylic acid. The epoxy resin has a molecular weight above 4,000 and provides from 50% to 90% of the initial reaction mixture, and the reaction takes place in the presence of benzoyl peroxide at an elevated temperature up to 130.degree. C., to effect addition polymerization of the monomer and to produce a graft polymer of addition polymer to the diepoxide. The reaction product may be dispersed in a basic aqueous medium, to form a water-reducible sanitary coating composition.
As is more particularly pointed out in patent application Ser. No. 788,611, the resinous reaction product produced contains three polymeric components, namely, the graft polymer, ungrafted diepoxide resin, and ungrafted addition polymer.
As is pointed out in Ser. No. 793,507, the initial epoxy resin, that is employed in the graft polymer production process, may be terminated to eliminate part or all of the terminal epoxy groups, so as to eliminate the possibility of ester grafting.
As is disclosed in these prior applications, in order to make acceptable water-reducible coating compositions, the addition polymerizable monomer comprises a major proportion of an unsaturated carboxylic acid, preferably either acrylic or methacrylic acid. Preferably, sufficient acid is employed so that the Acid Number (NV, i.e., based on non-volatiles) of the reaction product is from about 30 to 200, and preferably, 45 to 150. The acid functionality of compositions prepared in accordance with these several patent applications is derived from the acid functionality of the graft polymer component and of the ungrafted addition polymer component of the compositions. When the carboxyl groups are ionized by the addition of amine or other fugitive base to an aqueous vehicle containing such a composition, a stable dispersion is produced that is water-reducible. Such dispersions are stable over long storage periods even at somewhat elevated temperatures, and remain free from gelation and precipitation; only slight changes occur in pH levels and viscosities, indicating very little change in composition.
The reaction products of these prior applications appear to have remarkable properties. They are believed to serve as the means by which the ungrafted epoxy resin component is kept in stable suspension.
For sanitary coating applications, the preferred compositions are obtained from initial reaction mixtures in which the solids are derived 50% or more by weight from an epoxy resin having a molecular weight of about 4,000 or more, and the balance from addition polymerizable monomer of which the major proportion is acrylic or methacrylic acid. In a more preferred embodiment of a sanitary coating composition, the solids of the reaction mixture are derived from an epoxy resin that contributes from 60% to 90% and preferably about 80% by weight of the solids, the balance being a monomer mixture of methacrylic acid, styrene, and ethyl acrylate, where the acid is the predominant monomer. The preferred sanitary coating compositions produced from such reaction mixtures have Acid Numbers (N.V.) in the range from about 80 to about 90 and preferably about 85.
While resinous coating compositions of these kinds have excellent functional characteristics and other highly desirable properties, their high content of epoxy resin increases their cost. It would therefore be highly desirable to find some way to produce functionally equivalent materials, at lower cost.
Still another important consideration is the release of solvent materials into the atmosphere. In the process of making the reaction products of the patent applications described above, it has been customary to use liquid organic solvents, for the purpose of facilitating handling during the manufacturing process, and to improve application properties.
In the most preferred embodiment of the invention, for example, it has been customary to use two different solvents, a first solvent in which the epoxy resin, the graft polymer, and the addition polymer are all soluble, (although not necessarily in solution), and a second solvent that can dissolve the addition polymer product and that can solvate the addition polymer side chains of the graft polymer. These solvents remain with the resinous reaction product after it is inverted by the addition of water and a fugitive base. Consequently, when an applied coating is cured, which is usually accomplished by heating, the solvent is driven off and ordinarily escapes into the atmosphere. With the present concern over the release of organic solvent materials into the atmosphere, it is highly desirable that coating compositions be prepared in such manner as to reduce to the smallest feasible percentage the amount of organic solvent liquid present.